Light-sensitive electric device



/ Nov. 28, 1939. c, N|X 2,181,494

'LIGHT SENSITIVE ELECTRIC DEVICE Filed June 25, 1956 2 Sheets-Sheet lCONDUCTING METAL BASE THALLIUM THIN SEMI-TRANS PARENT LAYER OF A NOBLEMETAL SULFIDE OF THALLIUM INVENTOR F. C. N/X

ATTORNEY Nov. 28, 1939.

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RESPONSE IN OPEN CIRCUIT VOLTAGE SHORT GIRCU/T CURRENT ARE/TRAIN UNITSIN ARBITRARY UNITS IN ARB/TRARY UNITS F. c. NIX 2,181,494

LIGHT SENSITIVE ELECTRIC DEVICE 2 Sheets-Sheet 2 Filed June 25, 1936 70do 90 I 4b 5a 60 ILLUMINATION //v A 0 /0 20 4'0 6'0 so I07 lLLUM/NAT/ON//v A .60 /3 a WAVELENGTH IN MIGRONS z 8 a i Q 4 3 1.; 1g [6 2 a 24 Q 3050 I00 500 I000 5000 I0000 20000 FREQUENCY //v CYCLES 1 rap PER SECONDATTO NE! Y Patented Nov. 28, 1939 PATENT OFFICE LIGHT-SENSITIVE ELECTRICnEvIcE Foster 0. Nix, 'New York, N. 1., assignor to Bell TelephoneLaboratories,

Incorporated, New- York, N. Y., a corporation of New York ApplicationJune 25, 1936, Serial No. 87,231

13 Claims. (Cl. 136-49) This application relates to light sensitiveelectric devices and more specifically to photo-E. M. F. cells.

It is well known that when contiguous layers of certain materials,notably copper and copper oxide or selenium and. a suitable conductingmetal, are connected in an electric circuit and exposed to light anelectromotive force will be generated without the use of a separatesource of electromotive force in the circuit. Such light sensitive cellsare generally known as photo- E. M. F. or photo-voltaic cells.

This invention is based upon the discovery that contiguous layers ofthallium and thallium properly treated with sulphur provide a photo- E.M. F. cell having improved characteristics as compared with those ofother known photo- E. M. F. cells. In the preferred arrangement thethallium is treated with sulphur to produce thallous sulphide and thisisthe form of cell to which the following description primarily relates.

A metal base may be provided for the thallium and a transparentconducting film for the thallous sulphide. The thallous sulphide is thenexposed to light through the transparent film and contacts made to thecell through the transparent film and the metal base. This cell-has beenfound to give a current response from a tungsten filament lamp orsimilar light source having a maximum spectral response in the red orinfrared which response is many times that. obtained from any otherknown photo-E. M. F. cell. This cell also has other advantages whichwill be discussed more fully below.

One method of preparing such cells is by evaporating a layer of thalliumupon a metallic base member, which may be, for example, nickel, removingthe surface oxidation from the thallium layer by any suitable means suchas by heating it in vacuum, exposing the pure thallium surface to a glowdischarge inhydrogen sulphide (H28) to form a film of thallous sulphideon the thallium surface, cooling to room temperature, and then applyingto the thallous sulphide surface a thin, semi-transparent auxiliaryelectrode of sputtered metal such as platinum. As a modification, thindiscs of pure thallium may be used instead of evaporating the thin layerof thallium on a metal backing. As a further modification, thesulphurization process maybe carried out by vaporizing pure flowers ofsulphur instead of subjecting the thallium to a glow discharge in Hisgas.

The invention may be more readily understood by referring to thefollowing description taken in connection with the accompanying drawingforming a part thereof, in which:

Fig. 1 is a schematic diagram of a photo-E. M. F. cell of the type towhich this invention is directed; 5

Fig. 2 is'a perspective view of one of these cells in a suitableprotecting cover;

Fig. 3 is an exploded view of the cell of Fig. 2; Fig. 4 is a side view,partly in cross-section, of the cell shown in Fig. 2; 10

Fig. 5 is a side view, partly in cross-section of a cell having adifferent casing from that of the cell shown in Figs. 2 to 4; and Figs.6 to 9, inclusive, show characteristic curves of photo-E. M. F. cells ofthe kind de- 15 scribed above and illustrated in Figs. 1 to 5.

in one embodiment, a metallic disc or base mem- 20 ber ill of anysuitable rigid, conducting metal such as, for example, nickel or copperwhich serves as a conducting mechanical support for a layer ll ofthallium which is partially sulphurized to form a thin film l2 ofthallous sul- 25 phide upon which is sputtered a semi-transparentconducting film l3 of a suitable metallic material such as platinum.

The only members necessary to produce the electromotiveiorce are thethallium layer ll 30 and the thallous sulphide film l2 as the other twomembers are merely for the purpose of making electrical contact to thecell. It was observed in a large number of cells that a photo- E. M. F.effect was obtained by simply making contact with a platinum-pointedneedle at points on the illuminated thallous sulphide surface before thesputtering of the platinum auxiliary electrode 13 upon this surface.These observations clearly show that the presence of the sputtered filmis not essential to the observed photo- E. M. F. effect. The directionof flow of the photoelectrons in the cell during these observations wasalways from the thallous sulphide into 45 the free thallium metalfromwhich the thallous sulphide was obtained by sulphurization. Thearrow in Fig. 1 indicates the direction of flow of'the photoelectrons.An impedance such as, for example, that represented diagrammatically 50by the load Il may be placed in the external circuit to control thecurrent in that circuit.

The thallium layer may be a disc punched out of a thin sheet or it maybe a thin film evaporated upon the metal base member lll.

Cells in accordance with this invention have been made as follows:

When the thallium was evaporated upon the nickel or copper base memberdisc, the process was carried out in an evacuated glass tube, the basemember It! being mounted within the tube. Pure thallium metal (themethod of purifying the thallium will be described below) was placed ina suitable container within the tube from which it was evaporated ontothe metal disc, heat being supplied to the container by an electricheating coil. A thin layer H of thallium was thus formed on the basemember H).

In some cases it has been found preferable to prepare the thallium indisc form rather than to evaporate a thin layer of thallium upon a basemember. Thallium, which in its commercial form is usually in sticks orpellets, must be purified before it is suitable for use in-theseprocesses. The impurities, "which are essentially oxide coatings, wereremoved by melting in vacuum and allowing the molten metal to flowthrough several constrictions about 0.03 inch in diameter, thuseffectively straining out much of the oxide from, the thallium andleaving it in a clean condition. The thallium in this form was melted ina reducing flame and allowed to flow into smooth fiat bars, about 5 ofan inch thick. These bars were then rolled out into sheets about 0.021inch thick from which the discs used in making the cells were punched.Care must be taken in rolling out the thallium sheets to avoiddeveloping a flaky surface.

Whenever the discs were exposed to air, a thin layer of thallous oxidewas immediately formed on the thallium. This film was removed bydissolving it in distilled water or dilute acid. The discs were thenplaced in a disc-shaped aluminum holder mounted in a horizontal positionnear the bottom of a vertically mounted tube for the sulphurizationprocess. The process tube was immediately evacuated to prevent furtheroxidation. All of the remaining traces of oxide were then removed byheating. In general a temperature of 260 C. maintained for about twohours was found to be sufiicient to clean up the discs.

If after the heat treatment the thallium had a bright, metallic lusterit was ready for sulphurization. One method employed for sulphurizingthe thallium was to expose it to a glow discharge in an atmosphere ofhydrogen sulphide gas (HzS) at a temperature of about 260 C. Thistemperature is not critical as sensitive cells have been made when thetemperature was as low as C. and as high as 280 C. These temperaturesare both below and above the allotropic transformation point of thalliumwhich is about 230 G. However, up to the present time a temperature of260-140 C. has produced the most sensitive cells. This temperature maybe maintained by any suitable means, such as, for example, a cylindricalcoil or furnace around the containing tube, The glow dischargetook'place between two perforated aluminum plates about A; inch apartand located about 4 inch from the disc and parallel thereto. Thedistance between the plates and the distance of the plates from the discwas chosen so as to bring the disc just within the region of the glowdischarge so that a heavy concentration of sulphur atoms will beproduced near the thallium. The H25 gas was allowed to flow through thetube continuously at a rate which would maintain a gas pressure of about1.3 millimeters of mercury. The voltage impressed on the perforatedaluminum plates was about 390 volts. This gave a discharge current of 29milliamperes for the given pressure and size of plates. The time ofexposure to the discharge for best results was about three minutes.

A second method of sulphurizing the thallium consists in vaporizingflowers of sulphur (preferably degassed) to produce sulphur vapor andallowing the sulphur vapor to pass into the process tube wherein thethallium disc or layer is mounted. Before being used in the process, thesulphur may be boiled in vacuum and vaporized to remove all impurities.The side tube in which the purified sulphur was placed was joined to theprocess tube at a point intermediate the ends thereof. The process tubewas evacuated and then the thallium disc or layer was heated to atemperature of 260:10 C. and this temperature maintained throughout thesulphurization process. The sulphur was then heated to a high enoughtemperature to cause it to boil and the sulphur vapor thus allowed toflow into the proc-' ess tube and sulphurize the thallium, producing afilm of thallous sulphide. Some typical cells which have been made byone or the other of the processes stated above have been tested to'determine chemically the composition of the thin film produced by thesulphurization. In all cases the film was found to comprise TlzS(thallous sulphide). Possibly some additional sulphur may i be presentin the film, either as free sulphur or in the combined state as thallicsulphide ('IlS).

The appearance of the thallium surface rapidly undergoes numerouschanges as its treatment with sulphur progresses. First, a very thin,translucent black layer is formed. Addition of a small amount of sulphurto the unit in this state results in a sudden change such that the colorof the surface becomes a dark gray. This is believed to be the mostsensitive condition. By substantially increasing the quantity of sulphurintroduced into the reaction chamber a violent reaction may beprecipitated causing the surface to melt and then immediately solidify.In instances in which excessive amounts of sulphur were used acrystalline surface wasobtained. The most sensitive cells were found tobe those in which the surface had not melted. Those cells in which thesurface had become molten in spots during the sulphur treatment werefound to have high sensitivity but somewhat lower than the maximumsensitivity obtained from those of the preceding group; Use of slightlygreater quantities of sulphur such that the reaction was stoppedimmediately after the entire surface of the cells melted resulted in theproduction of cells of lower sensitivity. Still lower sensitivityresulted when sulphurization was heavy enough to give crystallinesurfaces.

Both methods of sulphurization described above have been found suitablefor preparing these cells as by both methods it is possible to maintaina constant and uniform sulphur concentration at the surfaces to besulphurized and to permit sulphurization to continue for accuratelycontrolled periods of time under repro-.

ducible conditions. It will be understood, however, that any othersuitable method for partially sulphurizing'the thallium may be used. Incomparing the two methods, in general, the hydrogen sulphide glowdischarge method is susceptible to more accurate control while thesulphur vaporization method is quicker and requires simpler apparatus.

After sulphurization the evacuated system is allowed to cool to roomtemperature at which time the unit is removed and receives a thinauxiliary electrode of sputtered platinum. Gold is also a suitablesubstance for this auxiliary electrode. The semi-transparent conductingauxiliary electrode l3 serves as the front electrode, and the metallicsupporting disc I constitutes the back electrode. The completed cellsmay then be placed in an evacuated glass vessel in order to protect themfrom changes in atmospheric conditions. The cells have proven to bequite stable over an observation period of several months.

Vacuum is not necessary to the successful operation of the completedphoto-E. M. F. cell herein described, but some container or casing is,necessary in order to protect the cell from changes in atmosphericconditions such, as, for example, an excess of water vapor in the air.Such a casing may be evacuated if desired Two typical casings will bedescribed below.

. Figs. 2, 3 and 4 show by way ofexample a thallous sulphide photo-E. M.F. cell in a typical molded container. assembled cell, Fig.3 is anexploded view of the cell and Fig. 4 shows the cell in cross-section.The complete assembly comprises a cell 'unit member I5, a front cover l6and a back cover I! together with means such as the screws 18 forconnecting the parts together. The cell shown in Fig. 1 comprising thebase member III, the thallium layer II, the thallous sulphide film l2and the semi-transparent platinum layer |3 may be set in a casing ofmolded ceramic material I!) on which may be placed a brass cover plate20 having an opening 2| therein. The brass plate 20 is fastened to theceramic material l9 by any suitable means such as the screws 22 and nuts23. The plate 20 may be slightly concave so that it forces the elementsof the cell unit in close contact'relation with each other. A contactlead 24 may be connected to the screw 22 by nuts 23 and 25 to form afront contact lead. The screws 22 also act as spacers in the assembly ofthe complete cell. The rear contact is made by a screw 26 whichmakeselectrical contact, through the ceramic material IS, with the base-member Ill; The contact lead 21 may be connected to the screw 26 by anysuitable means, such as by nuts 28 and 29.. To protect the unit fromchanges in humidity a thin coating 30 of a wax-like substance may beprovided for the transparent metallic layer l3. This coating 30 isplaced in the aperture 2| of the plate 20 and is transparent. Ozokeritewax and paraflin are satisfactory waxes for this purpose.

The front cover I6 for the cell unit may com prise a casing 3| of moldedmaterial such as, for

example, Bakelite, having a glass window 32 in front of the opening 2|in the metal plate 20. The casing 3| is adapted to completely enclosethe cell unit IS with the exception of the ends of the screws 22 and isprovided with threaded holes 33 to receive the screws H3.

The back cover-member l1 comprises a casingwhich is preferably of thesame material as that of the front casing 3|. It is provided withbinding posts 34 and 35 which terminate in screws 36 and 31 on theinside of the back casing member. The contact lead 21 is adapted to beconnected to the screw 36 and the contact lead Fig. 2 shows thecompletely the inner surface of the back cover I! press the cell unit l5close to the glass 32. When the device is assembled as described above,a change in the light intensity incident upon the glass member 32, andthrough this member upon the cell unit IE, will produce a difference ofpotential between the binding posts 34 and 35.

Fig. 5 shows a thallous sulphide cell having a metal container. Thiscontainer comprises a metal cylinder 40, of suitable material such asbrass, having a shoulder 4| at one end. A brass ring 42 rests on thisshoulder and is fastened to the cylinder 40 by any suitable means suchas solder. disc 32 which is sealed into the cylinder by any suitablemeans such as by Picien, a commercial form of sealing wax. A hard rubberring 43, having holes or indentations 44 to fit around the heads of thescrews 22 and hold the unit IS in place in the cylinder 40, is placedagainst the The leads 24 and 21 extend through the wax for contactpurposes. A hole 46 may be drilled in the cylinder 40 before the cell isassembled so that when the wax is poured in there is a pas.- sage forthe escape of air. This hole is sealed after the wax is poured by anyconvenient means such as solder. In the metal case cell, the coating 30of Ozokerite or other wax may not be necessary.

Measurements on cells similar to those described above at temperaturesvarying from room temperature down to the temperature of liquid air haveshown that the short-circuit current resulting from exposure to theradiation of an incandescent tungsten lamp is substantially proportionalto the incident light intensity at all temperatures. Fig. 6 shows thecharacteristic short-circuit current versus per cent il1umina-- -tion ofa-cell at -180 C'., the unfiltered light standard being taken as 100%and filters being used to produce the other intensities incident uponthe cell.

Fig. 7 shows. the open circuit voltage versus per cent illumination of atypical one of these cells. This characteristic departs from the linearform at higher percentages of illumination but the curvature is notserious, Many cells have been made in which this characteristic is alsolinear. The curves shown in Fig. 6 and Fig. '7 are typical of resultsobtained on a large number of cells.

The short-circuit current yield from cells of 4 Against the ring 42 isplaced the glass energy emission in the region of red and infrared suchas, for example, an incandescent tungsten lamp. This short-circuit yieldof 5000 microamperes per lumen may be compared with a yield from cuprousoxide front and back wall cells of 150 and 15 microamperes per lumenrespectively, and a yield from selenium photo- E. M. F. cells of about450 microamperes per lumen.

The open circuit voltage produced by cells of this type from a lightsource of approximately 0.37 lumen has reached a value as high as 0.1?volt. The average from this light source was .found to be around 0.11volt. These figures compare veryfavorably with the selenium photo- E. M.F. cell and are much larger than those of the cuprous oxide cells, bothfront and back wall types.

Curve I of Fig. 8 shows a typical equi-energy spectral response curve ofa thallous sulphide photo-E. M. F. cell. Curve 2 of this figure shows atypical transmission curve of thallous sulphite film on glass. Thislatter curve is included in order to show the relationship betweenoptical a direct comparison to be made between this characteristic ofthe two cells. As may be seen from the two curves, the thallous sulphidephoto- E. M. F. cell possesses vastly superior properties at highfrequencies to the selenium photo-E. M. F. cell or for that matter toany other known photo-E. M. F. cell.

Careful measurements were made in order to ascertain whetherrectification occurred in the cell such as is found in most cuprousoxide and selenium photo-E. M. F. cells. Some of these cells produceslight rectification while others do not, the rectification efiectapparantly being present in cells having relatively thick thalloussulphide layers. As pointed out above the cells having very thinthallous sulphide layers were found to give the best photo-E. M. F.response. These results tend to indicate that the rectificationphenomenon is not to be considered as a necessary accompaniment to thephoto-E. M. F. effect although both effects may exist in the same cell:

The cells have internal resistances of from 20 to 1500 ohms but thisresistance appears to be substantially independent of the intensity ofillumination. The cells cannot therefore be considered asphotoconducting.

It will be apparent that this thallium photo- E. M. F. cell possessesmany advantages over other known photo-E. M. F. cells. Variousmodifications may obviously be made without departing from the spirit ofthe invention, the scope of which is defined by the appended claims. Inthe claims, the word layer is intended to be broad enough to cover thearrangement wherein the thallium disc is used.

What is claimed is:

1. A photo-E. M. F. cell comprising a metal base a thallium memberadjacent to and in contact with said metal base, and a film of thalloussulphide on the face of said thallium member remote from the base.

2. A photo-E. M. F. cell comprising a metal base, a thallium layerdeposited on said base so as to be integral therewith, and a film ofthallous sulphide on said thallium layer.

3. A photo-E. M. F. cell comprising a metal base, a thallium layer onsaid base, a film of thallous sulphide on said thallium layer, and asemi-transparent electrically conducting layer of a noble metal on saidthallous sulphide.

4. A photo-E. M. F. cell comprising a base of conducting metal, athallium layer on said base, a film of thallous sulphide on saidthallium layer, and a thin film of a noble metal on said thalloussulphide.

5. The method of preparing a thallous sulphide photo-E. M. F. cellcomprising the steps of sulphurizing a layer of thallium until thesurface of the thallium attains a dark gray color, and applying a. thinlight-conducting layer of a noble metal to said sulphurized layer.

6. The method of preparing a thallous sulphide cell comprising the stepsof evaporating a film of thallium upon a metallic member, exposing thethallium surface to a glow discharge in H2S gas to form a film ofthallous sulphide on said thallium surface, cooling to room temperature,and applying to said thallous sulphide surface a. thin semi-transparentelectrode of a noble metal. 7. The method of preparing a thalloussulphide cell comprising the steps of sulphurizing a thallium disc byexposure to sulphur vapor in a temperature of approximately 260 C. toform a film of thallous sulphide on said, thallium surface, cooling toroom temperature, and applying to said thallous sulphide surface. a thinsemi-transparent electrode of a noble metal.

8. The method of preparing a thallous sulphide photo-E. M. F. cellcomprising the steps of sulphurizing a layer of thallium involving theapplication of heat until the surface of the thallium attains a darkgray color, cooling to room temperature, and then applying to saidsulphurized layer a conducting electrode of a noble metal.

9 The method of preparing a thalloussulphide cell comprising the step ofsulphurizing a thallium member by exposure to sulphur vapor at atemperature within the range from 250 C. to 270 C. to form a film ofthallous sulphide on said thallium.

10. The method of preparing a thallous su1- :phide cell comprising thesteps of sulphurizinga thallium member by exrmsure to sulphur vapor at atemperature within the range from 250 C. to 270 C. to form a film ofthallous sulphide on said thallium surface, and then cooling to roomtemperature.

11. The method of preparing a thallous sulphide cell comprising thesteps of sulphurizing a thallium member by exposure to sulphur vapor ata temperature within the range from 250 C. to 270 C. to form a film ofthallous sulphide on said thallium surface, cooling to room temperature,and sputtering on said thallous sulphide surface a thin semi-transparentelectrode of a noble metal. I

12. A photo-E. M. F. cell comprising a metal base, a thallium memberadjacent to and in contact with said metal base, and a film of amaterial comprising thallium and sulphur on the face of said thalliummember remote from the base.

13. A. photo-E. M. F. cell comprising a metal base, a thallium memberadjacent to and in contact with said metal base, and a film of asulphide of thallium on the face of said thallium member remote from thebase.

FOSTER C. NIX.

